The invention relates to a radiation curable colored coating composition and to said colored coating composition when applied to a coated optical fiber or when applied to at least one of a plurality of coated optical fibers assembled together in a ribbon.
Optical glass fibers are generally coated with two superposed radiation-cured coating layers, which together form the so-called primary coating or primary coating system. The coating layer (more briefly xe2x80x9ccoatingxe2x80x9d) which is in direct contact with the glass is called the inner primary coating and the overlaying coating, which is on the exposed surface of the coated fiber, is called the outer primary coating. The inner primary coating may also be called the primary coating; then, the outer primary coating is called the secondary coating. Both definitions are used interchangeably.
The inner primary coating is usually a relatively soft material while the outer primary coating is a relatively harder material. The primary coating system is designed to provide environmental protection to the glass fiber and resistance, inter alia, to the well-known phenomenon of microbending, which can lead to attenuation of the signal transmission capability of the fiber and is therefore undesirable. In addition, the primary coating system is designed to provide the desired resistance to physical handling forces, such as those encountered when the fiber is submitted to cabling operations.
In general, the primary coating system is applied onto the optical fiber during the drawing manufacturing process of the optical fiber.
In telecommunications applications of optical fibers, multiple individual strands of coated fiber can be packaged into larger structures such as ribbons and cables, to maximize efficiency. However, after ribboning and cabling of fiber, the individual strands of fiber must be readily distinguishable from each other so they can be accurately identified during, for example, installation and repair. Cable geometry and/or color coding can be used to distinguish and identify individual fibers in a complex cable.
Although several methods can be used to color code fiber, color coding can be done advantageously with either a thin colored layer (about 10 microns or less), also called an ink composition, which is placed over the primary coated fiber before cabling and/or ribboning of the same or by applying a colored outer primary coating onto the inner primary coating.
Typically, the application of the colored outer primary coating onto the inner primary coating takes place during the drawing process of the optical fiber. On the other side, the application of a colored layer onto the primary coated optical fiber generally takes place on a separate manufacturing line, after the primary coated optical fiber has been produced.
For the sake of conciseness, in the following of the present specification the term xe2x80x9cinternal coatingxe2x80x9d will indicate a coating disposed to surround the glass portion of the optical fiber, thus comprising either an xe2x80x9cinner primary coatingxe2x80x9d or a xe2x80x9cprimary coating systemxe2x80x9d (i.e. comprised of an inner and an outer primary coating). Said internal coating is then in turn coated with a colored coating. The terms xe2x80x9ccolored coating compositionxe2x80x9d, xe2x80x9ccolored layerxe2x80x9d, xe2x80x9cink layerxe2x80x9d and xe2x80x9cink compositionxe2x80x9d are used interchangeably throughout the specification.
Tape-like optical fiber ribbons are prepared by embedding at least two individual color coded fibers in a supporting matrix material which, like the inner and outer primary coatings, is also radiation-curable to maximize production speed. Optical fiber ribbons may comprise e.g., 4 to 12 colored fibers. The matrix material can encase the color coded optical glass fiber or the matrix material can edge-bond the glass fibers together. Cure of the matrix material occurs during the ribboning stage after the fibers have been color-coded by applying a colored coating. Hence, in a ribbon design, the ink layer resides between the ribbon""s matrix material and the fibers"" outer primary coating.
This means that the ink layer""s interfacial characteristics (e.g., surface energy, adhesion) must be carefully controlled to function properly with both matrix material and outer primary coating in the ribbon structure. In particular, the ability of a cured matrix material to be suitably stripped off the ink layer (break-out) is an important technical consideration. Ribbon break-out is generally carried out by a mechanical force, although chemical softening of the matrix with use of solvents is also known.
Optical fiber color coding can be based on up to 12 or more colors. Although optical fiber inks were originally solvent-based or thermosetting inks, in more recent times, radiation-curable inks have been used to increase the speed of the inking process. In these ink compositions, pigment is dispersed in a radiation-curable carrier or base composition.
As the demand for coated optical glass fibers has increased, manufacturers must respond by adding more fiber drawing production lines and by attempting to increase the linear line speeds of the existing fiber drawing/coloring production lines. In the latter case, one factor which will determine the upper limit for the line speed will be the curing rate characteristics of the radiation-curable ink composition, for a given radiation source and intensity.
If the line speed is increased to the extent that cure rate time requirements of the radiation curable ink composition are not provided, the radiation curable ink composition will not have received a sufficient amount of radiation to cause complete cure, or cross-linking, of the radiation-curable ink composition.
The production linear line speed is generally inversely related to the amount of radiation striking the optical glass fiber. That is, as the production line speed is increased, the amount of radiation exposure to the radiation-curable ink composition during the production process will necessarily decrease for a given radiation source. Incomplete cure of the radiation-curable ink composition is undesirable and must be avoided because then the desired properties of the ink coating may not be achieved and/or the incompletely cured ink coating may retain tackiness (giving problems in subsequent handling) or a malodorous odor may be present, and there may also be an undesirable increase of extractable components in the supposedly-cured ink coating.
In general, radiation-curable ink coating compositions cure at a significantly slower rate than radiation-curable outer primary coating compositions.
It is believed that the pigments present in ink compositions contribute to the slower cure speed of ink coatings. Thus, there is a need for improving the cure speed of the ink.
While the ink composition must have a very fast cure speed to ensure complete cure of the ink coating on the high speed drawing/coloring lines, the increase in cure speed should not come at the expense of other important properties of the ink coating, such as that providing suitable break-out performance. Break-out performance is the ability of the cured ink coating to separate from the matrix material without separating the ink layer from the outer primary coating, to provide an easy access to the individual coated optical glass fibers contained within the ribbon assembly, for instance during cabling/connection operations of the optical fibers.
Therefore, a radiation-curable ink composition should preferably exhibit adaptable adhesion properties to provide an adhesion between the outer primary coating and the ink coating that is greater than the adhesion between the ink coating and the matrix material to provide easy fiber access.
International Patent application Publication No. WO 98/50317 discloses a ribbon assembly comprising a colored optical fiber, wherein the colored coating of said optical fiber is formed from a radiation curable system which contains a mixture of oligomers, monomers and at least one photoinitiator, selected in such a way as to provide a level of adhesion between the ink coating and the matrix material which is less than the level of adhesion between said ink coating and the underlying inner coating of the optical fiber.
Patent application EP-A-614099 describes the use of a release agent such as a silicon oil or a fluororesin between the bundling layer and the coloring layer. In particular, when substantial amounts of silicone resins are used, incompatibility in the liquid and imperfections in the cured matrix composition may result, which causes attenuation of light.
Published Japanese patent application JP-A-01022976 describes a radiation curable ink composition comprising an alkoxylated bisphenol A diacrylate oligomer, a trifunctional reactive diluent and a homolytic photoinitiator.
The Applicant has now observed that, while some of the known ink compositions may satisfy the above different adhesion requirements, these inks generally have an insufficient resistance to water, in particular when a ribbon comprising the coated and inked optical fibers is soaked in water for a relatively long period of time. This characteristic is further called in the present specification the xe2x80x9cwater soak resistancexe2x80x9d of a fiber. Other of the known ink compositions, which may show the desired water soak resistance do not however fulfill the adhesion requirements.
In the present application, water soak resistance is referred to the capability of the fiber to maintain substantially unaltered its optical and mechanical parameters upon exposure to water. This property can advantageously be determined by measuring the variation of the attenuation value of the signal transmitted through an optical fiber immersed in water. In the following, when referring to the water soak properties of an optical fiber, the term xe2x80x9coptical fiberxe2x80x9d includes within its meaning either an optical fiber as such or an optical fiber disposed within a matrix material to form a ribbon of fibers. According to what is observed by the Applicant, fibers having good water soak properties are those wherein the attenuation value is substantially constant in time when the fiber is immersed in water at a predetermined temperature and for a predetermined time.
In particular, the variation of the measured attenuation value should be less than about 0.05 db/km for at least two weeks when the fiber is immersed into water at a temperature of 60xc2x0 C. As a matter of fact, as observed by the Applicant, fibers showing an increase of more than 0.05 db/km within less than two weeks of testing can not guarantee reliable optical performances during their entire operating life.
Although not wishing to be bound by any particular theory, it is believed that the increase in the attenuation value of an optical fiber immersed in water can be correlated to the fact that water may penetrate at the interface between two coating layers, thus determining possible microbending phenomena which may cause an increase in the attenuation of the transmitted signal.
The Applicant has further observed that while a fiber coated with a colored layer may show good water soak performances when tested as a single fiber, the same fiber may have unacceptable properties when coated with a matrix material to form an optical fiber ribbon. As observed by the Applicant, the interface between the colored layer and the matrix layer is thus the most critical interface for the water soak properties of optical fiber ribbons. Therefore, a relative good adhesion between the colored layer and the matrix layer should be achieved and maintained during the entire operating life of the optical fiber, in order to avoid water penetration at the interface of these two layers.
There is thus an apparent incompatibility between the requirement of good release properties and the requirement of good water soak properties. While the first property requires a relatively low degree of adhesion between the colored layer and the matrix, the second property requires a rather good adhesion between the two layers, which would not be affected by decay due to water presence.
Having recognized the above problem, the Applicant has now found that it is possible to optimize the release properties and the water soak properties of the optical fiber, in particular when said optical fiber is disposed within an optical fiber ribbon, by suitably formulating the composition of the resin which is applied as the colored coating in order to achieve acceptable values of both these properties.
One aspect of the present invention thus relates to a radiation curable colored coating composition for coloring a coated optical fiber wherein the coating, when disposed and cured to surround an optical fiber coated with an internal coating, and when said colored fiber is coated with a matrix material and assembled into an optical fiber ribbon:
said colored coating has a degree of adhesion to the internal coating which is higher than the degree of adhesion to the matrix material;
and said optical fiber assembled into said optical fiber ribbon shows, upon aging for at least two weeks in water at 60xc2x0 C., an increase in the attenuation of the transmitted signal at 1550 nm of less than 0.05 db/km with respect to the attenuation of the assembled optical fiber measured before aging.
A further aspect of the present invention relates to a colored coating composition for coloring a coated optical fiber wherein the coating, when disposed and cured to surround an optical fiber coated with an internal coating, and when a plurality of said colored fibers are bound together by a matrix material to form an optical fiber ribbon, said colored coating layer has a degree of adhesion to the internal coating which is higher than the degree of adhesion to the matrix material, said degree of adhesion to the matrix material being however sufficiently high such that said optical fibers show, upon aging for at least 2 weeks in water at 60xc2x0 C., an increase in the attenuation of the transmitted signal at 1550 nm of less than 0.05 db/km with respect to the attenuation of the optical fibers measured before aging.
Preferably, the increase in the attenuation of the transmitted signal at 1550 nm is less than about 0.05 db/km, upon aging of the assembled fiber for at least one month in water at 60xc2x0 C. More preferably, the fiber is aged in water at 60xc2x0 C. for at least two months without showing said attenuation""s increase, particularly preferred being an aging of at least four months without showing said attenuation""s increase.
Preferably, said internal coating comprises an inner primary coating and an outer primary coating and the colored coating has a thickness of from about 3 to about 10 microns.
A further aspect of the present invention relates to a radiation curable colored coating composition comprising:
(A) 40-60% by weight of a bisphenol A epoxy diacrylate, a modified bisphenol A epoxy diacrylate or a mixture of both,
(B1) 15-30% by weight of an alkoxylated aliphatic glycol diacrylate diluent,
(B2-5-25% by weight of a trifunctional acrylate diluent,
(C) 6-20% by weight of a photoinitiator system consisting of at least two different homolytic free-radical photoinitiators, and less than 4% by weight of benzophenone
(D) 1-9% by weight of a polydimethylsiloxane based silicone release agent, and
(E) 1-15% by weight of a dry pigment, and comprising less than 5% by weight of a urethane acrylate, wherein, if an optical fiber is coated with an internal coating and with said colored coating, and if said colored fiber is coated with a matrix material and assembled into an optical fiber ribbon, said optical fiber shows, upon aging for at least two weeks in water at 60xc2x0 C., an increase in the attenuation of the transmitted signal at 1550 nm of less than 0.05 db/km with respect to the attenuation of the assembled optical fiber measured before aging.
Preferably, the two homolytic free-radical photoinitiators of component (C) of said radiation-curable colored coating composition differ in their respective photosensitivity.
Preferably, said radiation curable colored coating composition further comprises less than 3% by weight of N-vinyl caprolactam.
Preferably, said radiation curable colored coating composition comprises as the trifunctional acrylate diluent (B2) trimethylol propane triacrylate.
According to a particularly preferred embodiment of the present invention said radiation curable colored coating composition consists essentially of:
(A) 40-60% by weight of a bisphenol A epoxy diacrylate, a modified bisphenol A epoxy diacrylate or a mixture of both,
(B1) 15-30% by weight of an alkoxylated aliphatic glycol diacrylate diluent,
(B2) 5-25% by weight of trimethylol propane triacrylate,
(C) 6-20% by weight of a photoinitiator system consisting of at least two different homolytic free-radical photoinitiators, and less than 4% by weight of benzophenone
(D) 1-9% by weight of a polydimethylsiloxane based silicone release agent, and
(E) 1-15% by weight of a dry pigment.
Preferably, the alkoxylated aliphatic glycol diacrylate diluent (B1) of said radiation curable colored coating composition is ethoxylated aliphatic glycol diacrylate.
Further, component (D) of said radiation curable colored coating composition preferably is a non-reactive polydimethyl siloxane based silicone release agent.